1. Field of the Invention
The present invention relates to ducted turbines and, more particularly, to a ducted turbine which is both rugged and exceptionally efficient, resulting in high torque at low fluid (such as wind) speed and quiet operation. In a first preferred embodiment, increased efficiency is obtained by a structure which accelerates the fluid (such as wind) passing through the turbine duct and slows fluid passing through the outer diameter of the turbine. In a second preferred embodiment, increased efficiency is obtained by placing a fluid redirection device (a vortex enhancer) on a ducted turbine to direct fluid passing through the duct towards the outer diameter of the turbine.
2. Discussion of Related Art
Increases in energy costs, the finite supply of fossil fuels such as oil, natural gas, and coal, and pollution caused by burning fossil fuels have prompted the search for efficient and clean energy alternatives to fossil fuels. One alternative which has received widespread attention is the use of wind or air flow to generate energy. Legislation exists in many parts of the country requiring local utilities to accept and pay for inputs from such alternative energy sources, heightening the practicality and interest in implementing this source of renewable energy.
Another advantage of wind energy is that it may generate energy at locations not easily accessible to remotely generated energy, such as locations where no power line infrastructure exists. Such locations may be, for example, remote or underdeveloped areas, and ocean vessels. Thus, it may be advantageous to generate energy at a particular location where it is not feasible to supply the energy via a cable, wire, or other means. An efficient wind turbine may provide this advantageous arrangement. For example, applications such as water pumping and desalinization at remote areas and electrical power generation for ocean vessels, offshore rigs, and other locations to which it may be impossible or impractical to run power lines may benefit greatly from a highly efficient wind turbine. Moreover, wind energy does not create pollution and thus is clean.
Unfortunately, most wind turbine systems currently available have several principal drawbacks which make them impractical for most localities. The first and most important of these drawbacks is that the typical wind turbine does not operate efficiently unless there is a relatively high wind velocity, often as high as twenty knots. Many areas of the world do not have sustained wind velocities of more than six to ten knots, causing most common wind turbines to be unsuitable for use as a reliable energy source.
The second principal drawback is the high noise level produced by most commonly available wind turbines. The blade arrangement of these turbines is usually designed to obtain maximum revolution rate, resulting in a disturbing, audible noise level which varies in intensity and pitch with variations in wind velocity.
The third drawback of some currently available wind turbines is the inability to withstand high velocity winds greater than forty knots. Unfortunately, velocities well in excess of forty knots occasionally occur in many parts of the world.
The fourth drawback is that turbine systems currently available have a limited efficiency. One believed limitation on turbine efficiency is a physical principal called the Betz Limit. The Betz Limit states that the maximum efficiency of the very best wind turbine (or other fluid turbine) is 59.3%.
U.S. Pat. No. 4,427,343 (the '343 patent) describes a highly efficient wind turbine for airflow velocities as low as three knots and which maintains its efficiency and structural integrity at higher airflow velocities. The wind turbine described in the '343 patent is illustrated in FIGS. 1, 2, 3A, and 3B. Low speed tests performed in 1991 on the '343 patent wind turbine suggest that its efficiency approaches the Betz Limit.
FIG. 1 shows a blade arrangement of a preferred embodiment of the ducted wind turbine 100 described in the '343 patent. The wind turbine 100 comprises a hollow cylindrical blade support mechanism 101, several helical rows of blades, such as the first, second, and third helical rows of blades 102, 103, and 104 seen in FIG. 1, and a power takeoff mechanism, such as a pulley 105. The hollow cylindrical blade support mechanism 101 includes the duct 250 (see FIG. 2) through which wind (or other fluid) may pass unimpeded by turbine blades.
The blades are attached at one end to the blade support mechanism 101 and extend radially outward therefrom, as well as extend in rows along the surface of the blade support mechanism such as rows 102, 103, 104. These rows spiral back in a rotational direction from the front 109 of the turbine, forming an angle with the axis of rotation 108. For a six row device, for example, this angle with the axis of rotation 106 is 24.degree..+-.5.degree.. The nominal diameter of the cylindrical support mechanism remains one-third of the wind turbine diameter as measured from diametrically opposed blade tip to blade tip. A practical embodiment may include twenty blades in a row. Only three rows are shown in FIG. 1 for clarity. However, the general spiral configuration of the rows is evident.
FIG. 2 is a front elevational view of the '343 patent wind turbine 100, showing a first row of blades 103 and a portion of a second row of blades 102. The remaining rows are similar and are not shown. The cylindrical support defines the duct 250. Both the interior and exterior surfaces of the duct are cylindrical. The exterior surface is the blade support surface, the interior surface is the central duct. The cylindrical support 101 may be supported by spokes 212 which extend radially from a hub 201. The spokes 212 are designed to have a relatively small frontal cross section to reduce drag and provide for an essentially unresisted flow through the duct. Between rows, there is preferably a nominal spacing 210 of 5.degree..
FIG. 2 shows the relative dimensions of the blades and the cylindrical support mechanism 101. This arrangement provides a relatively large circumferential area about the cylindrical support mechanism on which to mount the blades and eliminates masking, eddy currents, and weakened structures encountered in other prior art devices which use the hub for mounting the blades.
The blade arrangement described above provides high efficiency at low velocity. FIG. 3A shows a plan or edge view of a blade 300. FIG. 3B shows a front elevational view of the blade 300. The rear face 304 of the blade 300 has a curved, convex, or airfoil shaped contour which provides a "pulling" or lifting effect from the rear of the blade in addition to the normal direct pressure or "push" effect on the frontal area or forward concave face of the blade, thereby increasing the effective force delivered to the blade 300 by a passing fluid flow. FIG. 3B shows a front view of a blade 300. The blade 300 tapers from a wide width W2 303 at the tip to a narrower width W1 302 at the base.
Although the wind turbine described in the '343 patent is efficient, quiet, and rugged, improvements in efficiency remain desirable. Efficiency improvements make turbines more desirable because more usable energy per unit of fluid (such as wind) energy may be generated than previously possible.
It is an object of the present invention to improve upon the efficiency of the turbine described in the '343 patent and other ducted turbines.
It is yet a further object of the present invention to provide a wind turbine which has an efficiency exceeding those previously thought possible according to the Betz Limit.
It is even a further object of the present invention to provide a clean and efficient energy source.